Because of you are using the FormRequest in your Controller method, you can use the $requets->all(), $request->only('yourInput'), ... to manage it.
Try it:
$request->replace(array('inputname' => 'new value'));
//or
$request->merge(array('inputname' => 'new value'));
Here, in the official repository you can find in the Request class the replace method:
/**
* Replace the input for the current request.
*
* @param array $input
* @return void
*/
public function replace(array $input)
{
$this->getInputSource()->replace($input);
}
/**
* Merge new input into the current request's input array.
*
* @param array $input
* @return void
*/
public function merge(array $input)
{
$this->getInputSource()->add($input);
}
https://github.com/laravel/framework/blob/master/src/Illuminate/Http/Request.php
Hope it helps you.